Portable batteries with increased energy and power densities are required as the use of portable electronic equipment rapidly continues to increase. Batteries are typically the limiting factor in the performance of most commercial and military portable electronic equipment due to the restrictions on the size, weight and configuration placed by the equipment on the power source. In some cases, safety and environmental factors are also significant considerations for deploying a particular power source. Lithium batteries provide high energy density, conformal packaging and improved safety, which make them one of the most promising electrochemical systems under development today.
Lithium batteries use high valence metal oxide materials, which are reduced during the electrochemical reaction. This reaction in rechargeable lithium and rechargeable lithium ion batteries must be fully reversible in order to have a viable cell. Common reversible metal oxide materials used in lithium batteries include: LixMn2O4, LixCoO2 and LixNiO2. These materials remain reversible whenever “x” is maintained between 0.10 and 0.85 for LixMn2O4 and 0.4 and 0.95 for LixCoO2 and LixNiO2. However, if the stoichiometry exceeds these limitations, the material undergoes a phase change and is no longer reversible. The primary consequence of the phase change of the material and subsequent irreversibility is the cell will no longer accept a charge rendering the cell inoperable. In order to maintain this stoichiometry rigid electronic control is employed. Thus there has been a long-felt need to solve the problems associated with loss of reversibility in lithium batteries without suffering from the disadvantages, limitations and shortcomings associated with rigid stoichiometry electronic control and loss of reversibility. A mixed metal oxide that introduces potassium into the cathode structure yields a material that provides a high voltage cut off, which prevents over charge and thus retains reversibility.
In order to resolve the reversibility problem, electrochemical measurements were performed on rechargeable lithium batteries using potassium doped manganese dioxide as the positive electrode. These measurements identified two distinct reversible regions for the Li/LixKyMn2O4 electrochemical couple. Changes in cell behavior as a function of potassium stoichiometry in MnO2, as well as cell discharge and charge properties with respect to the potassium and lithium stoichiometry, were also measured. Preliminary results indicated that Li/LixKyMn2O4 electrochemical cells would produce the required reversibility and still meet other significant lithium battery operational objectives, without suffering from the setbacks, limitations and disadvantages of rigid stoichiometry electronic control and loss of reversibility associated with other lithium battery configurations.
The present invention provides a potassium-doped mixed metal oxide cathode material comprising alloying MnO2 with potassium and lithium in a LixKyMn2O4 compound affording overcharge protection that allows the cathode to be fully reversible. The LixKyMn2O24 material is incorporated into an electrochemical cell with either a lithium metal or lithium ion anode and an organic electrolyte. In one embodiment, the cathode of the present invention comprises a compound with the general formula LixKyMn2O4, where y<0.5, x+y<1.0 and the reversible region for x is between 0.0<x<1.0−y to provide the required overcharge protection and a high voltage cutoff on charge. In the preferred embodiment, a cathode comprising Li0.8K0.1Mn2O24 is provided. The cathodes of this invention answer the long-felt need for a reversible cathode for rechargeable lithium batteries without suffering from the shortcomings, limitations and disadvantages of, rigid stoichiometry electronic control and loss of reversibility. The present invention also includes a single step and two step method for making cathode material for lithium electrochemical devices.